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Muller Engineering Company, Inc. Happy Canyon Creek Major Drainageway Plan - DRAFT 5.5 Alternative Costs Cost estimates for the alternatives were developed using UDFCD’s master planning cost estimating spreadsheet, UD-MP Cost, Version 1.1. The spreadsheet includes set unit costs for various capital improvement items based on 2009 costs with an adjustment for inflation; the annual inflation rate assumed was 3.0%. Unit costs are shown in Table F-1. Capital improvement costs include set allowances for mobilization (5%) and for stormwater management and erosion control (5%), and user-defined costs for dewatering, traffic control, and utility relocation. Dewatering was set at 2.5% of the capital improvement cost; lump sums for traffic control were added for crossing improvements. No utility relocation was assumed. Allowances for other costs were added as shown in Table 5-3. No right of way costs for access and maintenance easements were included. Proposed improvements are assumed to be mutually beneficial to private landowners and local jurisdictions; in such cases, easements are generally granted when the improvements are constructed. 5.6 Alternative Plans Table 5-3 Additional Cost Allowances Item % of Capital Improvement Cost Engineering 15% Legal/Administrative 5% Contract Admin/Construction Management 10% Contingency 25% 5.6.1 Detention Alternatives Detention Alternatives that were considered are shown in Figures 5-1 through 5-3, and include existing and proposed detention facilities located on or adjacent to Happy Canyon Creek as well as on several tributary channels. Where the impact of a specific pond was in question, the facility was evaluated by running the SWMM model with and without the proposed improvement; alternatives that were considered but discarded due to limited impact are indicated. Potential new facilities or improvements to existing facilities were then grouped into three overall detention alternatives based on the type of improvements provided in each one. Peak inflow and outflow rates and storage volumes at each pond for each alternative are listed in Table F-2 alongside baseline hydrology and historic values. Peak flow diagrams for the 100-year, 10-year, 2-year, and water quality storm events are included as Figures F-1 through F-4; each diagram includes existing and future baseline hydrology, historic flow rates, and the three detention alternatives. Costs for detention alternatives were determined using the unit costs and multipliers established in UD-MP Cost. In order to effectively tabulate the costs for each individual facility in each alternative, the calculations were performed in a separate spreadsheet. Costs for new or fully reconfigured detention facilities were estimated using a complete-in-place unit cost per acre-foot, which would include excavation and embankment, outlet structures, maintenance access, overflow spillways, forebays and low-flow channel improvements, and revegetation. While this approach was applied uniformly, experience indicates that the cost to volume ratio should decrease for larger facilities; therefore, the total estimated costs for large detention facilities may be more conservative than for smaller facilities. This is especially true for the 100-year online peak-shaving ponds, which would only require a downstream embankment and large culvert crossing in order to construct as planned. Costs for retrofits were calculated based on varying assumed costs for outlet works improvements, with an allowance for revegetation where appropriate. Detailed cost estimates for each detention alternative are included in Appendix F. Detention Alternative A This essentially represents a “no-action” scenario, reflecting only planned full spectrum detention that will be publically owned and maintained. This includes facilities in the Sierra Ridge development, under the oversight of the Sierra Ridge Metropolitan District, in the RidgeGate development, under the oversight of the Rampart Range Metropolitan District, and in Compark South in the Town of Parker. The Grandview Pond is located in Sierra Ridge immediately upstream of Lincoln Avenue, on the Grandview Tributary. Final design has been completed and construction plans approved by Douglas County. Storage and discharge curves are based on updated design plans dated May 2012. A full-spectrum detention facility is proposed on the Green Acres Tributary in Compark South, at the location of the E-470 pond. As described earlier, the existing pond is not a formal pond but merely a large culvert crossing under E-470 that was described as a pond in the 1993 OSP. The conceptual drainage plan for the development indicates a water quality/100-year detention pond at this site; for this analysis, the sizing has been revised to reflect full spectrum detention for the contributing area within Compark South and within Grandview Estates. Two distinct types of ponds are shown within the RidgeGate development: offline EURV ponds and online 100-year peak-shaving ponds. Initially, a few large offline full spectrum detention ponds were assumed within this development. These ponds had been sized through iteration to match the existing conditions hydrology peak flow rates on Happy Canyon Creek and Badger Gulch. With approval of the project sponsors, additional analysis was done to determine the impact of offline detention on peak flow timing in the channel. The analysis modeled offline FSD ponds in comparison with offline EURV ponds that bypass the 100-year peaks, and found that the offline FSD ponds delay 100-year peaks such that their timing then corresponded with the channel peak, thereby causing an overall increase in the 100-year channel peaks. As a result of this analysis, the approach to detention within RidgeGate was changed to avoid placing 100-year detention offline, and instead utilizing online peak shaving ponds to reduce 100-year peaks. While online detention facilities are not generally recommended in this study in order to allow sediment movement within the channel, the peak shaving ponds are proposed to be configured as an embankment with a large culvert that August 2013 Page 5-6 Alternative Analysis
Muller Engineering Company, Inc. Happy Canyon Creek Major Drainageway Plan - DRAFT produces some flow attenuation in large events but allows smaller flows to pass through. Because these facilities are located within the floodplain, they are expected to have minimal impact on developable area. A total of 10 offline EURV ponds and two online 100-year ponds are shown; however, the actual number and location of facilities can be adjusted as needed based on the layout and timing of development. The number and placement is not significant to the results, as long as the offline ponds allow flows in excess of the EURV to pass unrestricted and the online ponds are configured to allow sediment movement in the channel. The capital improvement cost associated with Alternative A estimated using the methods described above is $18.3 million, with a total project cost of $28.4 million. As mentioned, these costs are viewed as conservative, especially in regards to the online 100-year ponds in RidgeGate. Detention Alternative B Alternative B includes the planned facilities described in Alternative A, and adds retrofits to existing detention facilities and formalization of existing detention volume. Retrofits to CPNMD ponds #9, 10, 11, and 20 are included, with each sized to provide EURV for the upstream contributing area. CPNMD Pond #12 is modeled with conveyance improvements to prevent flooding of the adjacent sanitary sewer lift station. These improvements consist of lowering the existing overflow spillway embankment by approximately 2’ to provide 1’ of freeboard in the 100-year event; this results in a loss of 3.7 acre feet of storage. The cost for these improvements is not included in the detention costs; because it is for the purpose of improving conveyance rather than providing additional detention, it is instead reflected in the channel alternative costs. Retrofits to two sub-regional facilities in Arapahoe County are reflected in the cost estimates for Alternative B, however, the ponds are not included in the SWMM model as their contributing areas are too small to be eligible for inclusion in master plan hydrology. SEMSWA has indicated that these ponds do not perform well, and retrofits to improve their function are desired. In addition to retrofits, the formalization of existing detention volume was explored for this alternative. An existing stock pond is located on the Beverly Hills Tributary immediately upstream of the confluence with Happy Canyon Creek; during field investigation of this pond, no evidence of overtopping was seen. No outlet to the pond was found (or is expected), which indicates that peak flows from the Beverly Hills Tributary seldom reach Happy Canyon Creek. Though this pond may not be the only reason for this phenomenon, it seems reasonable to incorporate this pond into the hydrology so that it’s benefit can be reflected in the downstream peak flow rates. The pond is located on Douglas County property and was modeled based on the existing storage volume and overflow spillway, with a 2” diameter outlet pipe assumed to reflect infiltration of the volume below the spillway. To formalize this pond, a geotechnical investigation of the embankment is recommended; an outlet structure could be provided if desired (costs are based on the standard $45,600/af, which is quite conservative for this case). Two additional existing facilities were considered for this alternative. The Chambers and Meridian reservoirs are reflected in the baseline hydrology as brim full with 100% imperviousness over the permanent pool. In actuality, these ponds typically operate with freeboard that would allow rain falling on the surface and on the areas around the pond to be captured. The SWMM model was modified to reflect such a condition at each pond to determine the potential benefit of an operating agreement establishing a minimum freeboard: in the 100-year event, there was zero benefit to Happy Canyon Creek from the Meridian Reservoir and less than 1% benefit from the Chambers Reservoir. These were therefore discarded as alternatives. Resulting peak flow rates for Alternative B are nearly identical to Alternative A in the 100-year event as the retrofits are aimed at providing EURV where 100-year detention already exists. Peaks are markedly reduced in the 10-year and 2-year events, which drop below existing along much of the channel. Peak flow rates for the water quality event are approximately half of the Alternative A peaks. The capital improvement cost associated with Alternative B is $18.7 million, with a total project cost of $29.0 million. The incremental project cost above the “no action” Alternative A is $0.6 million. Detention Alternative C The maximum detention improvements are reflected in Alternative C, which includes the planned and retrofitted facilities in Alternatives A and B as well as several new facilities and further modification to existing facilities. For Alternative C, CPNMD Pond #12 is assumed to be reconstructed with a new embankment located just upstream of the sanitary lift station; this configuration provides additional volume to reach the calculated EURV for the upstream contributing area while eliminating the undesirable levee condition at the sanitary sewer lift station. The volume needed to reduce 100-year peak flow rates to pre-development levels cannot be provided at this facility; Alternative C also reflects further restriction of the 100-year outlets at CPNMD Pond #11 in order to detain the 100-year peak discharge to levels below pre-development conditions. This partially offsets the higher peaks downstream while still providing substantial freeboard (over 8 feet) to Castle Pines Parkway. A new facility on the Beverly Hills Tributary at Charter Oaks Drive is included in this alternative. There are two existing crossings of Charter Oaks from two flow paths that converge on the downstream side; an existing detention pond is located at the east crossing and provides 10-Yr/100- Yr detention for the large shopping complex upstream and for the existing elementary school to the east. The west crossing is unrestricted, and the land on this side is owned by the Castle Pines North Metro District and is planned as a park. A middle school is currently under construction on the high August 2013 Page 5-7 Alternative Analysis
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TO: Study Sponsors and Stakeholders
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DRAFT ALTERNATIVE ANALYSIS COMMENTS
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Arapahoe County Comments Section 5.
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FINAL ALTERNATIVE ANALYSIS COMMENTS
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